netfilter: nf_tables: Support wildcard netdev hook specs

User space may pass non-nul-terminated NFTA_DEVICE_NAME attribute values
to indicate a suffix wildcard.
Expect for multiple devices to match the given prefix in
nft_netdev_hook_alloc() and populate 'ops_list' with them all.
When checking for duplicate hooks, compare the shortest prefix so a
device may never match more than a single hook spec.
Finally respect the stored prefix length when hooking into new devices
from event handlers.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>